Origin and geochemical evolution of groundwater in a closed-basin clayey aquitard, Northern Mexico

Groundwater flow cross-sectional modeling in combination with major ion and trace element chemistry, stable isotopes of water (O-18 and H-2) and geochemical modeling were used to investigate the origin and evolution of groundwater in a Quaternary lacustrine clay-rich aquitard in the closed basin of the 'La Laguna Region', Northern Mexico. This study pertains to the Viesca Lake, one of a series of ephemeral ancient lakes that existed in this basin until the early 1900s. The Viesca aquitard overlies a regional marine Mesozoic sequence that outcrops in the surrounding mountains. Groundwater samples were collected from springs, pumping wells in a carbonate aquifer and from former industrial production wells in the aquitard. Numerical results indicate a classical gravity flow system, where the carbonate aquifer discharged about 90% of the groundwater, as springs, that fed the former Viesca Lake. Isotope content of groundwater, in the carbonate aquifer, shows that groundwater has a local meteoric origin; and its chemical evolution is mainly associated to progressive dissolution of gypsum and oxidation of pyrite along the flow paths. In contrast, groundwater in the aquitard presented enrichment of heavy isotopes 2 H and 180, indicating that this water resulted from the evaporation of the carbonate groundwater aquifer. The maximum concentrations of sulfate (77,700 mg L-1), chloride (45,400 mg L-1) and sodium (45,000 mg L-1) in the groundwater from the aquitard were between three and four orders of magnitude greater than the fresh groundwater. Sulfate behaved as a conservative ion, contrasting with concentration of sulfate in groundwater from other aquitards in the world; which is usually reduced. Chemical patterns and enrichment of stable isotopes in groundwater from the aquitard follow general evaporation trends observed for other closed or semi-closed basins worldwide. Redox related parameters (NO3-N, NH4+, Mn2+, total Fe and TOC) and redox-sensitive trace elements (As, Se, Sb, Sn, Cr, and Bi) in groundwater indicated dominant aerobic conditions in the carbonate aquifer and anaerobic conditions in the clayey aquitard. (C) 2003 Elsevier B.V. All rights reserved.